The invention concerns a multi-channel device for the measurement of variable magnetic fields produced by various field sources, having field strengths of less than 10-.sup.10 T, and in particular of less than 10-.sup.12 T. The measuring device includes in each channel a superconducting quantum interference device, a gradiometer consisting of superconducting coils with predetermined dimensions and superconducting connecting elements between the quantum interference device and the gradiometer. In addition, the device includes electronic equipment for the evaluation, processing and presentation of the information obtained at the quantum interference devices.
The use of superconducting quantum interference elements, which are generally known as "SQUIDs" (abbreviation for "Superconducting Quantum Interference Devices"), for measuring very weak magnetic fields is generally known (see for example, "J. Phy. E:Sci. Instrum.," Vol. 13, 1980, pages 801 to 813; and "IEEE Transactions on Electron Devices," Vol. ED-27, No. 10, October 1980, pages 1896 to 1908). The preferred field of application for these devices is medical engineering, and in particular for magnetocardiography and magnetoencephalography. The magnetic cardiac or brain waves that occur in these sectors have field strengths that are located in the range of 50 pT to 0.1 pT ("Biomagnetism-Proceedings, Third International Workshop on Biomagnetism, Berlin 1980," Berlin/New York 1981, pages 3 to 31).
A device for the measurement of magnetic fields of this kind includes the following principal components:
1. A SQUID acting as a current sensor; PA1 2. A flux transformer with a coil arrangement acting as a field-to-current transducer for sensing the field; PA1 3. Electronic devices for collecting and processing signals and for output; PA1 4. Screening for the geomagnetic field and external interference fields; and PA1 5. A cryogenic system for the superconducting components.
Measuring devices of this type are generally know (see for example, S.H.E. Corporation, Dan Diego, USA/S.H.E. GmbH, D-5100 Aachen).
In corresponding measurement devices with a onechannel design, the magnetic field to be investigated is coupled inductively through a coil arrangement made of superconducting wire, into a circuit consisting of radiofrequency (RF) SQUID with a Josephson contact. Gradiometers of the first or higher orders are constructed by combining one sensor coil with one or more compensation coils. With gradiometers of this type, it is possible, with the right method of manual adjustment, to suppress almost entirely the three components of a homogenous magnetic field in the vicinity of the coils or of the portion of such a field with homogenous gradients. Additionally, the biomagnetic near field which is still strongly heterogenous in the vicinity of the gradiometers can be selectively obtained. The RF SQUID is also inductively coupled to a tank circuit, whose high-frequency voltage is modulated in phase or amplitude by the input signal. In general, the operating poing of the RF SQUID is maintained by negative feedback by means of an additional compensation coil, and the compensation current is used as a signal to be evaluated electronically.
using this familiar one-channel measuring device as a starting point, multi-channel devices have also been suggested. In this case, each channel has, in addition to a SQUID, a superconducting gradiometer and connecting elements between the SQUID and the gradiometer with a coupling transformer and connecting lines.
With the gradiometer coils it is possible to monitor the flux of the field vector of a field source that is to be observed, such as, for example, the "magnetic heart vector" ("Journal of Magnetism and Magnetic Materials," Vol. 22, 1982, No. 2, pages 129 to 201). This field vector is, however, dependent on distance. By adjusting the dimensions of the gradiometer coils with regard to the distance from the field source in each case, it is possible to optimize the sensitivity: that is, for a predetermined distance between a field source and a gradiometer coil, certain optimal dimensions for that gradiometer coil exist.
The object to be investigated in magnetic cardiac diagnostic, has a surface of about 100 cm.sup.2, and is located not at a single depth, but at depths ranging from 2 to 10 cm below the thoracic wall; furthermore, its location and dimensions are subject to periodic fluctuation. Accordingly, the practical problem arises from the fact that a gradiometer coil at the upper surface of the thorax which has a predetermined dimension, for example, a fixed diameter of 3 cm, can generally pick up only the near field of a small area of the front wall of the heart. It is unable to pick up the far field of the areas lying deeper within the body. Similarly, with the gradiometer coils that are used for magnetoencephalographic investigations, which have a diameter of about 2 cm and are optimally suited for measurements of field sources in the cerebral cortex, it is impossible to make measurements of sources in the brain stem. This is because for measurements of this kind, the optimum diameter of the gradiometer coil would be about 10 cm. Therefore, any adjustment of the coil dimensions to the specific distance to the field source to be investigated is virtually impossible in these conventional devices.